Laminated metal sheet and process for producing the same

ABSTRACT

When laminating two types of film ( 1, 2 ) with different melting points on the two sides of a metal sheet ( 3 ), the thickness of the low melting point side film ( 2 ) is adjusted. Specifically, the thickness d 2  of the low melting point side film ( 2 ) at the part sandwiched between the lamination roll ( 10 ) and metal sheet ( 3 ) is made a range defined by d 2≧ k(ΔMP−ΔT)/V. Here, ΔMP is the difference of melting points of the two types of film, k is k≧2, 0&lt;ΔT=MP 1−Φ Ti≦50(° C.) (Ti is the metal sheet temperature at the inlet side of the rolls, V is the sheet running speed, Φ is a constant determined by the heat removal conditions at the time of lamination, where 0.75≦Φ&lt;1). Due to this, sticking of the low melting point side film ( 2 ) to the lamination roll ( 10 ) can be prevented.

TECHNICAL FIELD

The present invention relates to a laminated metal sheet used as amaterial for food cans and other containers and comprised of a metalsheet laminated on the front and back with two types of resins withdifferent melting points and a method of production of the same.

BACKGROUND ART

As materials for food cans, beverage cans, aerosol cans, and othercontainers, metal sheets covered on their surfaces with a polyester,polyolefin, or other thermoplastic resin are being made much use of. Inthis case, the metal sheet used is generally steel sheet or aluminumsheet. Among these, for example, polyester-based resins are generallysuperior in corrosion resistance, flaw resistance, and printability andcan be used for both the inner surface and outer surface of cans in somecases, but when the content is alkaline, the resin is insufficient indurability. Further, when used for meat-based food cans, there is theproblem that the meat releasability is poor. To solve this problem,two-sided laminated metal sheets covered with different types of resinat the can inside surface side and the can outer surface side are beingused.

In general, as the can outer surface side film, a relatively hardpolyester-based resin film is preferably used, while as the can insidesurface side film, a lower melting point, excellent meat releasabilityand alkali durability, relatively soft polyolefin-based resin film ispreferably used. Metal sheet laminated with different films on its twosurfaces is disclosed in Japanese Patent Publication (A) No. 63-231926,Japanese National Publication (A) No. 2-501644, and Japanese PatentPublication (A) No. 2002-120324. Note that the terms “high melting pointfilm” and “low melting point film” used in the present description donot mean films with melting points of absolute values. A relatively highmelting point side film is called a “high melting point film”, while arelatively low melting point side film is called a “low melting pointfilm”.

In general, a laminated metal sheet is produced by the method ofsuperposing a heated metal sheet and resin films and using laminationrolls to apply pressure to bond them (heat lamination method). To makethem bond, the temperature of a film surface contacting the metal sheethas to be at least its melting start point Tsm (normally a temperatureabout 0 to 30° C. lower than the melting point), more preferably atleast the melting point MP, but on the other hand if the temperature ofa film surface contacting a lamination roll becomes the melting startpoint or more, the film will stick to the lamination roll makingproduction impossible.

For this reason, the temperature of the metal sheet at the laminationpart has to be strictly controlled in relation with the film meltingpoints, but when using a polyester-based resin film as the high meltingpoint film and using a polyolefin-based resin film as the low meltingpoint film, since the melting points MP of the two greatly differ (forexample, polyethylene terephthalate (polyester-based) has a meltingpoint of 265° C., while polypropylene (polyolefin-based) has one of 168°C.), there is the problem that if setting the temperature of the metalsheet to match with one of the films, the other film will not bond well.

Therefore, as shown in Japanese Patent Publication (A) No. 63-231926,the method of first laminating the high melting point resin film, thenlaminating the low melting point resin film in a later step where thetemperature of the metal sheet falls, that is, a two-step laminationmethod, and, as shown in Japan National Publication (A) No. 2-501644,the method of reheating after the lamination step by the press-bondingof the lamination rolls so as to melt-bond the resins have beenproposed, but both of these have the problems of swelling capital costs.Further, Japanese Patent Publication (A) No. 2002-120324 proposes asimultaneous lamination method matching the temperature of the metalsheet with the low melting point resin film, but it is believed that alow melting point resin for bonding purposes is required at the metalsheet side of the high melting point resin film, so again the costrises.

Further, in the prior art disclosed in Japanese Patent Publication (A)No. 63-231926 and Japanese Patent Publication (A) No. 2002-120324, sincethe low melting point resin film is kept from melt-bonding with thelamination roll by setting the temperature of the metal sheet to matchwith the low melting point resin film, the crystallization degree willnot be lowered to an extent enabling the laminated low melting pointresin film to be sufficiently worked. For this reason, when bending atwo-sided laminated metal sheet to work it to a food can etc., thephenomenon of the worked part of the low melting point film whiteningappears. It looks like the can contains foreign matter. Therefore, usersand can makers would be liable to raise complaints.

DISCLOSURE OF THE INVENTION

The present invention solves the above problems of the prior art andprovides a laminated metal sheet enabling simultaneous lamination of twotypes of film with different melting points, without sticking on thelamination rolls, on the two surfaces of a metal sheet and a method ofproduction of the same. Further, the present invention provides moreinexpensively a two-sided laminated metal sheet free from the lowmelting point resin film whitening even when working the sheet toproduce a food can etc. The present invention has as its gist thefollowing.

(1) A laminated metal sheet obtained by laminating two types of filmincluding a high melting point film and low melting point film on thetwo sides of a metal sheet, said laminated metal sheet characterized byhaving a low melting point film thickness d2 defined by the followingequation 1:d2(μm)≧k(ΔMP−ΔT)/V

where, ΔMP=MP1−MP2

MP1: melting point of high melting point film (° C.)

MP2: melting point of low melting point film (° C.)

k: constant determined by low melting point film's heat conductivity,heat capacity, and temperature, k≧2 [μm·m/(° C.·s)]0<ΔT=MP1−Φ·Ti≦50 (° C.)

Ti: metal sheet temperature at lamination roll inlet side (° C.) Φ:constant determined by heat removal conditions at time of lamination(0.75≦Φ<1)

V: sheet running speed (m/s)

(2) A laminated metal sheet as set forth in (1), characterized in thatsaid high melting point film is comprised of a polyester-based resin andsaid low melting point film is comprised of a polyolefin-based resin.

(3) A laminated metal sheet as set forth in (2), characterized in thatsaid polyester-based resin is selected from polyethylene terephthalate,a polyethylene terephthalate/isophthalate copolymer, polyethylenenaphthalate, a polyethylene terephthalate/naphthalate copolymer, a mixedresin of polybutylene terephthalate and polyethylene terephthalate, amixed resin of polybutylene terephthalate and a polyethyleneterephthalate/isophthalate copolymer, and further one of these resinsincluding a pigment or dye.

(4) A laminated metal sheet as set forth in (2), characterized in thatsaid olefin-based resin is selected from a polypropylene, polyethylene,a polypropylene/polyethylene random copolymer, or these resins includinga pigment or dye.

(5) A laminated metal sheet as set forth in (1), characterized in that adifference ΔHz between a haze value Hz2 of a low melting point filmafter bending a sheet of a thickness of 1 mm by 180 degrees across saidhigh melting point film side and a haze value Hz1 of the low meltingpoint film before bending is made not more than 20%.

(6) A laminated metal sheet as set forth in (5), characterized in thatthe haze value Hz1 of the low melting point film before bending is madenot more than 60%.

(7) A laminated metal sheet as set forth in (1) or (2), characterized byusing for said high melting point film a biaxially stretchedpolyester-based resin film and leaving crystal orientation at thatsurface layer.

(8) A laminated metal sheet as set forth in (1), characterized in that,among the two types of film including said high melting point film andlow melting point film, when the melting points inside the films differaccording to the position in the thickness direction, the lower surfacemelting point of the one film at the side contacting the roll isdesignated as MP2 and the film melting point of the other film at themetal sheet side is designated as MP1 for calculation and lamination.

(9) A laminated metal sheet as set forth in (8), characterized in thatthe sheet running speed V defined in (1) is made 1 to 3.5 m/s forlamination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged sectional view of a two-sided laminated metalsheet of the present invention.

FIG. 2 is an explanatory view of a process of production of a two-sidedlaminated metal sheet.

FIG. 3 is a view of the temperature profiles immediately before therolls separate from the films in the case of laminating same meltingpoint films on the two sides of a metal sheet.

FIG. 4 is a graph of a heat conduction analysis model.

FIG. 5 is a graph of temperature gradients inside the two types of film.

FIG. 6 is a graph of a method for finding a thickness by which a filmwill not stick to the rolls.

FIG. 7 is a view of the temperature profiles at the lamination roll exitside in the present invention.

FIG. 8 is an explanatory view of bending by 180 degrees.

FIG. 9 is a perspective view of an easy peel EOE.

BEST MODE FOR WORKING THE INVENTION

FIG. 1 is an enlarged sectional view of a two-sided laminated metalsheet of the present invention, wherein 3 indicates a steel sheet,aluminum sheet, or other metal sheet, 1 a high melting point filmlaminated at one side of this metal sheet 3, and 2 a low melting pointfilm laminated at the other side of this metal sheet 3.

FIG. 2 is an explanatory view of a process of production of a two-sidedlaminated metal sheet according to the present invention. In FIG. 2, 10indicates a pair of left and right lamination rolls. A high meltingpoint side film 1 and a low melting point side film 2 are superposedover the two sides of the metal sheet 3 and press bonded by thelamination rolls 10. The metal sheet 3 is for example a steel sheet. Inthe case of use as a material for a container, the metal sheet 3 has athickness of 0.1 to 0.5 mm or so in usual cases. The high melting pointside film 1 is, for example, a relatively hard, superior printability,flaw resistance, retort bondability, etc. polyester-based resin, whilethe low melting point side film 2 is, for example, a superior corrosionresistance, workability, meat releasability, retort bondability, etc.polyolefin-based resin. Note that A indicates a point immediately beforethe metal sheet 3 and films contact, while B indicates a pointimmediately before the films separate from the lamination rolls 10.

In the present invention, the metal sheet 3 is heated in advance to atemperature at least the melting start point of the high melting pointside film 1, preferably the melting point+50° C. or less, for example,270° C. As a result, both the film 1 and the film 2 are raised intemperature at the sides contacting the metal sheet 3 to at least themelting start points and bond with the metal sheet 3. At this time, boththe films 1 and 2 rise in temperature at the opposite sides (sidescontacting the lamination rolls 10) due to conduction of heat from themetal sheet 3, but the film 1 can generally be bonded under conditionswhere the roll exit side metal sheet temperature Td (=Φ·Ti, Φ being aconstant determined by the heat removal conditions at the time oflamination, 0.75≦Φ<1) is less than the melting start point of the film1, so under those conditions, will not stick to the lamination roll 10.

FIG. 3 schematically shows, for explanatory purposes, the temperatureprofiles inside the metal sheet and films immediately before the rollsseparate from the films in the case where the same melting point filmsare bonded to the two sides of the metal sheet 3. The temperature of themetal sheet Ti at the point A immediately before the metal sheet and thefilms are brought into contact by the rolls becomes higher than themelting start point Tsm1 of the films, but the temperature of the metalsheet Td (=Φ·Ti) at the point B immediately before the rolls separatefrom the films usually becomes lower than the melting start point Tsm1of the films. Therefore, the temperatures of the film surfaces at thispoint of time become lower than the melting start point Tsm1 of thefilms and the films will not stick to the rolls 10.

Usually, each film has at thickness determined by the reason that forexample the lower limit of film formation is 10 μm or that for corrosionresistance to be maintained, 20 μm or more is necessary. By selectingthe lamination conditions in accordance with the determined thickness,it is possible to avoid sticking. The conditions under which a film willnot stick to a roll are conditions where, while the roll and film are incontact, the temperature at the point C of the film thickness positionis less than the melting start point Tsm1. This can be achieved byselecting the lamination conditions. Specifically, when laminating twotypes of film with different melting points on a metal sheet, ifconsidering corrosion resistance in food can applications, the highmelting point side film generally has a thickness of 20 to 30 μm or so,but greater thicknesses can be obtained for other applications.

Further, the temperature of the metal sheet when contacting the filmsnormally becomes higher than the melting start point Tsm1 of the films,but at the point of time when the rolls separate from the films, asshown in FIG. 3, it becomes lower than the melting start point Tsm1 ofthe films. This is because heat is removed through the film to the rollside. If considering the fact that the temperature (Ti) at the point Aimmediately before the metal sheet and films contact is higher than themelting start point Tsm1 of the films, by experience, as shown inequation 1, 0<MP1−Td=MP1−Φ·Ti≦50 (° C.) becomes required as a condition.That is, if MP1−Φ·Ti becomes 50° C. or more, Ti becomes Tsm1 or less andbonding is sometimes not sufficient. This condition becomes a requiredcondition of the roll inlet side temperature for bonding the highmelting point film to a metal sheet when laminating two types of filmwith different melting points. Note that Φ is determined by the heatremoval conditions at the time of lamination and specifically isdetermined by adjusting the lamination rolls in surface temperature orpressing force. Φ<1. In general, 0.75≦Φ<1 can be adjusted to.

Further, the higher the temperature Ti at which the metal sheet andfilms are bonded, the better the bondability of the metal sheet and thehigh melting point film. If raising Ti, Td (=Φ·Ti) also becomes higher,so to further raise the bondability, by experience, it is sufficient toset Ti about 10° C. higher than the bonding lower limit temperature.This can generally be achieved if 0<MP1−Φ·Ti≦40 or so. By raising thefilm bondability, the film peeling strength, corrosion resistance,retort bondability, etc. can be improved.

When laminating the high melting point film 1 and low melting point film2, if applying lamination conditions commensurate with the melting pointof the film 1 without considering the film thickness, due to the heatconduction from the metal sheet 3, the temperature of the film 2 at theside contacting the lamination roll 10 will exceed the melting startpoint of the film 2 and the film will end up sticking to the laminationroll 10 in some cases. However, in the present invention, the lowmelting point side film is adjusted in thickness d2 to specifically maked2≧k (ΔMP−ΔT)/V and thereby solve this problem. Here, ΔMP=MP1−MP2, whereMP1 is the melting point (° C.) of the high melting point film, MP2 isthe melting point (° C.) of the low melting point film, and V is thesheet running speed (m/s). Further, ΔT=MP1−Φ·Ti.

Here, k is a coefficient expressed by equation 2 according to the Law ofHeat Conduction: $k = \frac{\lambda_{2}L}{{Cp}_{2} \cdot \alpha}$

Below, the content of equation 2 will be explained. First, as shown inFIG. 4, a one-dimensional heat conduction analysis model of the insideof a film having an abscissa x indicating the film thickness and anordinate indicating the temperature T is prepared. The temperature whenx=0 is the temperature of the metal sheet at a certain time t. Further,if the temperature of the side of the film contacting the roll is lessthan the melting start point Tsm1, it is assumed that the film will notstick to the roll. The distance d not sticking to the roll becomes thenecessary film thickness.

In the present invention, two types of film are laminated on the metalsheet, so, as shown in FIG. 5, two curves are shown, but the heatconductivities λ and the specific heat CP do not greatly differ (seldomdiffer by an order of magnitude), so the distance x is divided by λ andthe result multiplied with Cp to obtain a parameter and the two curvesare approximated as the single curve as shown in FIG. 6. Further, inFIG. 6, by finding the coordinates (dCp/λ)1 and (dCp/λ)2 of theintersections of the horizontal lines drawn from Tsm1 and Tsm2 and thiscurve, multiplying these values with λ1 (λ2), and dividing by Cp1 (Cp2),the thicknesses d1 and d2 sticking to the rolls can be calculated.

If approximating the curve of FIG. 6 by a straight line, it is expressedby T=A−B (Cpx/λ). When x=0, T=Td=Φ·Ti=A, so T=Φ·Ti−B (Cpx/λ). Therefore,to prevent roll sticking, Φ·Ti−B (Cpx/λ)≦Tsm, therefore, x≧λ/(Cp·B)(Φ·Ti−Tsm). Right now the problem is the conditions under which the lowmelting point side film will not stick to the roll, so d2≧λ2/(Cp2·B)(Φ·Ti−Tsm2) The time t is the contact time, so t=L/V (V is the sheetrunning speed, and L is the contact length). If considering the factthat the inclination B is inversely proportional to the contact time(when the contact time becomes longer, the temperature becomes even),B=α (V/L). Therefore, d2≧λ2·L/(Cp2·α·V) (Φ·Ti−Tsm2). If λ2·L/(Cp2·α)=k,then d2≧k (Φ·Ti−Tsm2)/V.

This d2≧k (Φ·Ti−Tsm2)/V is substantially equivalent to d2≧k (ΔMP−ΔT′)/Vfrom the relationships Tsm1−Tsm2≅MP1−MP2=ΔMP and ΔT′=Tsm1−Φ·Ti. SinceΔT=MP1−Φ·Ti≧ΔT′, d2≧k (ΔMP−ΔT′)/V≧k (ΔMP−ΔT)/V. As shown in FIG. 7, thisexpresses the conditions for maintaining the temperature of the lowmelting point side film 2 at the side contacting the lamination roll 10at less than the melting point of the film 2. Note that in FIG. 3, asimplified temperature gradient is shown, but the time during which thefilms 1 and 2 pass the lamination rolls 10, 10 is a short time ofseveral tens of msec. Nonconstant heat movement occurs, so accurateanalysis of the actual heat movement phenomenon is extremely difficult.Further, the pressing forces of the rolls, the surface conditions of thefilms, etc. cause the state of contact of the metal sheet and films andthe films and roll to change. Due to this, the films also change insurface temperatures, so the temperature gradient also changes.Therefore, when finding the range of the value of k based on the Law ofHeat Conduction operatively, it was learned that the range was k≧2. Thelarger the value of k, the greater the effect of suppression of filmsticking, but actually 5 or so is sufficient.

Here, the method of finding d2 will be explained again.

1) Two types of film are selected.

2) The low melting point side MP2, λ2, and Cp2 are investigated.

3) The high melting point side MP1 and λ1 are investigated.

(If there are results of normal operation, λl and Cp may be unknown)

4) The value of Ti and the conditions of Φare found from MP1.

Normally, when MP1−Φ·Ti=ΔT, ΔT≦50° C.

5) d2 is found from d2≧k(ΔMP−ΔT)/V.

In this case, the minimum value of d2 is when k=2. This value isdetermined with reference to λ2 and Cp2 of the low melting point film 2and the operating conditions. Note that the value of d1 is determined bythe corrosion resistance and other functions.

Further, according to the above formula, if making the sheet runningspeed V greater, the films can be made thinner in thickness d2, but forthe runnability of the films or the uniform heating of the metal sheet,it is not preferable that the sheet running speed V be made that high.Normally, the operation is performed at 2.5 to 3.5 m/s or so. However,depending on the facility, it is of course possible to make the valueone over 3.5 m/s. As the metal sheet, various types of metal generallyused as container materials such as aluminum sheet, soft steel sheet,various types of plated steel sheet, stainless steel sheet, etc. may beused.

The resin forming the high melting point film 1 may in principle be anythermoplastic resin. Ones selected from polyethylene terephthalate,polyethylene terephthalate/isophthalate copolymer, polyethylenenaphthalate, polyethylene terephthalate/naphthalate copolymer, a mixedresin of polybutylene terephthalate and polyethylene terephthalate, amixed resin of polybutylene terephthalate and a polyethyleneterephthalate/isophthalate copolymer, or these resins including apigment or dye are frequently used. Normally, for increasing thestrength or hardness, a biaxially stretched film is used.

Further, for the purpose of increasing the bondability, it is alsopossible to give the metal sheet surface a chromate coating, give thesurface of the resin contacting the steel sheet an adhesive layer havingpolarity, etc. in combination.

On the other hand, the resin forming the low melting point film 2 may inprinciple be any thermoplastic resin. Resins selected frompolypropylene, polyethylene, a polypropylene/polyethylene randomcopolymer, or these resins including a pigment or dye are frequentlyused. These polyolefin-based resins are lower in melting point comparedwith the polyester-based resin forming the high melting point film 1.For example, the polyethylene terephthalate resin used as the highmelting point film 1 has a melting point of 265° C., while thepolypropylene resin used as the low melting point film 2 has a meltingpoint of 168° C.

In the above way, the two-sided laminated metal sheet of the presentinvention is one in which the low melting point film 2 is adjusted inthickness to become thicker and is simultaneously laminated underrelatively high temperature conditions matching the temperatureconditions of the metal sheet 3 with the high melting point film 1. As aresult, the laminated low melting point film 2 is heated to aconsiderably higher temperature than the conventional two-sidedlaminated metal sheet and becomes amorphous.

For this reason, the two-sided laminated metal sheet of the presentinvention becomes resistant to whitening even when bending for workingit into a container etc. Clarifying this point by numerical values, inthe present invention, as shown in FIG. 8, a sheet 4 having a thicknessof 1 mm is bent 180 degrees to the outside across the high melting pointfilm 1 side. The whitenesses of the low melting point film 2 before andafter this are defined by the haze values. That is, the bending work is180 degree bending so that the low melting point film 2 becomes theoutside.

The haze value is the value defined as the diffusion transmittance/totallight transmittance×100 (%) when optically measuring the haze value ofthe film. The measurement method is prescribed in JIS-K7136. Here, thetwo-sided laminated metal sheet before bending and after bending isimmersed in 40° C. 18% HCl to make the metal sheet 3 dissolve, the lowmelting point film 2 is peeled off, then a 50 mm×50 mm sample formeasurement of the haze is taken so that the bent part becomes thecenter, the measurement is performed three times, and the average valueis taken. The measurement is performed centered at the bent part.

The two-sided laminated metal sheet of the present invention has adifference ΔHz=Hz2−Hz1 of not more than 20% when the haze value of thelow melting point film 2 before bending is Hz1 and the haze value afterbending is Hz2. Further, the haze value Hz1 of the low melting pointfilm 2 before bending is preferably not more than 60%. By making thisdifference of the haze value ΔHz before and after bending not more than20%, even when performing bending work etc. in the food can productionprocess, the low melting point film 2 is free from whitening and the canno longer appears to contain foreign matter.

Note that the whitening at the time of the bending work is accompaniedwith the formation of microcracks, but if ΔHz is made not more than 20%as shown in the following examples, this problem can be avoided.Further, if the haze value Hz1 of the low melting point film 2 beforebending is over 60%, the inner surface of the can whitens in appearance,so this is not preferable.

In the above way, the two-sided laminated metal sheet of the presentinvention has a difference ΔHz of the haze values of the low meltingpoint film 2 before and after bending of not more than 20% and can beproduced by the low production cost simultaneous lamination method.Further, the temperature conditions of the metal sheet 3 can be setmatching with the characteristics of the high melting point film 1, soit is possible to use for the high melting point film a biaxiallystretched polyester-based resin film and laminate it in the state withthat surface layer having residual crystal orientation.

Note that when working the two-sided laminated metal sheet of thepresent invention into a container body, the high melting point film 1is usually made the outside and the low melting point film 2 the inside.However, when using as the easy peel EOE (easy open end) shown in FIG. 9or other can inside lid 5 the two-sided laminated metal sheet of thepresent invention, to raise the heat sealability with the outer lid 6laminated with polypropylene at its bottom surface, sometimes the lowmelting point film 2 is used as the top side (can outer surface side).In this way, which side to make the outside of the container may besuitably determined in accordance with the application.

EXAMPLE 1

The various types of film 1 (melting point MP1, thickness d1) and thevarious types of film 2 (melting point MP2, thickness d2) shown in Table1 were laminated under the lamination conditions shown in Table 1 on thetwo sides of chrome plated steel sheets. Invention Examples 1 to 8 allhad thicknesses d2 of the films 2 larger than the values of d2calculated from the lamination conditions. The roll sticking of thefilms 2 of the invention examples and the bondability of the films 1 tothe steel sheet were evaluated. The results are shown in Table 2.Further, examples outside the conditions of the present invention aresimilarly shown as Comparative Examples 1 to 5 in Table 1 and Table 2.

Note that, in Table 1, PP shows polypropylene film, but pure PP isnonpolar and cannot sufficiently be bonded by heat lamination, so onegiving the metal bonding surface side a modified PP adhesive layer(melting point 166° C.) having a thickness of 4 μm and polarity is used.Further, the polyethylene film shown as PE is also nonpolar and cannotbe sufficiently bonded by heat lamination in the pure state, so onegiving the metal bonding surface side an ethylene acrylate copolymerresin adhesive layer (melting point 99° C.) of a thickness of 10 μm isused. TABLE 1 d2 calculated MP1 d1 MP2 d2 V Ti ΦTi value No. Film 1 ° C.μm Film 2 ° C. μm m/s ° C. Φ ° C. k μm 1 Inv. Ex. 1 PET-IA*5 226 20 PP*7168 40 2.5 233 0.858 200 2 25.5 2 Inv. Ex. 2 PET-IA*5 226 20 PP*7 168 252.9 233 0.858 200 2 22.0 3 Inv. Ex. 3 PET-IA*5 226 20 PE*8 112 80 2.5257 0.778 200 2 70.4 4 Inv. Ex. 4 PET*6 265 13 PP*7 168 70 2.5 285 0.842240 2 57.6 5 Inv. Ex. 5 PET-IA*5 226 13 PP*7 168 85 2.5 226 0.931 210 584.8 6 Inv. Ex. 6 PP*7 168 20 EAA* 99 65 1 173 0.751 130 2 61.8 7 Inv.Ex. 7 PET*6 265 20 PP*7 168 40 3.5 271 0.849 230 2 35.5 8 Inv. Ex. 82-layer PET 226 13 PP*7 168 30 2.5 234 0.855 200 2 25.7 9 Comp. Ex. 1PET-IA*5 226 20 PP*7 168 25 2.5 233 0.858 200 2 25.5 10 Comp. Ex. 2PET-IA*5 226 20 PP*7 168 20 2.5 203 0.862 175 2 5.6 11 Comp. Ex. 3PET-IA*5 226 20 PP*7 168 20 2.5 245 0.922 226 2 46.3 12 Comp. Ex. 4PET-IA*5 226 20 PE*8 112 60 2.5 257 0.778 200 2 70.4 13 Comp. Ex. 5PET*6 265 13 PP*7 168 40 2.5 285 0.842 240 2 57.6*EAA: ethylene acrylate copolymer

TABLE 2 Film 2 roll Film 1 No. sticking bondability Remarks 1 Inv. Ex. 1OK OK Plenty of leeway 2 Inv. Ex. 2 OK OK Increase of speed avoidssticking to lamination roll 3 Inv. Ex. 3 OK OK Change of type of film 4Inv. Ex. 4 OK OK Change of type of film 5 Inv. Ex. 5 OK OK k practicalupper limit 6 Inv. Ex. 6 OK OK V = 1.0 m/s 7 Inv. Ex. 7 OK OK V = 3.5m/s 8 Inv. Ex. 8 OK OK 2-layer structure 9 Comp. Ex. 1 NG OK Filmsticking limit 10 Comp. Ex. 2 OK NG High temperature side bonding limit11 Comp. Ex. 3 NG OK ΦTi = MP1 12 Comp. Ex. 4 NG OK Change of type offilm 13 Comp. Ex. 5 NG OK Change of type of film

According to Invention Example 1, even when performing retort processingin steam at 125° C.×30 minutes, a good bondability free from filmpeeling is obtained. Comparative Example 1 is an example in which theother conditions are made the same as in Invention Example 1, but thethickness of the film 2 is made 25 μm or smaller than the 25.5 μm of thecalculated value of d2 and sticking to the lamination roll occurred.Therefore, an example in which the other conditions are made the same,but the sheet running speed of Comparative Example 1 is raised to 2.9m/s is Invention Example 2. Due to this, the calculated value of d2becomes 22 μm or less than the 25 μm of the thickness of the film 2. Asa result, sticking to the lamination roll can be avoided.

Comparative Example 2 is an example of lowering Ti to Td=ΦTi=175° C. andas a result making ΔT=MP1−ΦTi 51° C.

In this case, bonding is no longer possible. Further, ComparativeExample 3 is an example in which ΦTi is raised to 226° C. or equal tothe melting point MP1 of the film 1. The film 2 stuck to the laminationroll.

Invention Example 3 is an example of changing the type of the film 2from Invention Example 1 to PE. A laminated metal sheet was producedwithout allowing sticking to the lamination rolls. Comparative Example 4is an example in which the conditions were made the same as in InventionExample 3 and the thickness of the film 2 was made 60 μm or thinner thanthe 70.4 μm of the calculated value of d2. Sticking of the film 2 to thelamination roll occurred.

Comparative Example 5 is also an example in which the thickness of thefilm 2 was made 40 μm or thinner than the calculated value of d2.Sticking of the film 2 to the lamination roll occurred. Therefore, inInvention Example 4, the other conditions were made the same as those ofComparative Example 5 and the thickness of the film 2 was made 70 μm. Asa result, the film no longer stuck to the lamination roll. Even ifretort sterilizing the obtained laminated metal sheet, the film 1 didnot peel.

Invention Example 5 is an example where the thickness of the film 2 ismatched with the calculated value of d2. This is an example where thethickness of the low melting point film is close to the upper limit. Ifcalculating the k value of this case utilizing the 85 μm thickness ofthe low density polyethylene film, about k=5. This extent of k value isbelieved to be the actual upper limit.

Invention Example 6 is an example where even if the sheet running speedwas made the lower limit of 1 m/s, lamination was possible withouttrouble. Further, Invention Example 7 is an example in which even if thesheet running speed is made the 3.5 m/s upper limit value of thefacility, lamination was possible without trouble. Invention Example 8is an example of use of a two-layer PET film as the film 1. Thistwo-layer PET film is comprised of an outside of PET of a thickness of 7μm and a melting point of 265° C. and an inside of PET-IA (*5) of athickness of 6 μm and a melting point of 226° C.

In the above invention examples and comparative examples, as the metalsheet, chrome plated steel sheet was used, but unplated steel sheet,aluminum sheet, copper sheet, etc. treated on their surfaces as needgive similar results as confirmed from experiments.

Note that *1 to *8 in the evaluation column of Table 1 show thefollowing content.

*1: Ia/Ib is the ratio of the following two peaks obtained by X-raydiffraction measurement using CuKα-rays at a two-sided laminated metalsheet at the high melting point film (polyester film) covered side. Iais the X-ray diffraction intensity of the diffraction face of the (100)face parallel to the surface of the polyester film (distance betweenfaces about 0.34 nm), while Ib is similarly the X-ray diffractionintensity by the diffraction face of the (110) face (distance betweenfaces about 0.39 nm). In the examples, Ia/Ib is 0.7 to 10 in range andshows that the crystal orientation remains without complete melting upto the surface layer.

*2: The corrosion resistance after working is the result from punching atwo-sided laminated metal sheet into a disk shape of a diameter of 158mm, drawing it by a draw ratio of 1.56 so that the low melting pointfilm becomes the inner surface to obtain a shallow drawn cup, thenredrawing it by a draw ratio of 1.23 to obtain a can of a cup diameterof 82 mm and a cup height of 52 mm (DRD can), filling the inside with 2%citric acid, then storing it at 37 degrees for six months and examiningthe inside surface of the can for the state of corrosion.

*3: PET-PBT is a mixed resin of polyethylene terephthalate andpolybutylene terephthalate.

*4: Two-layer PET is two-layer structure film with a surface layer of 10μm PET (MP 265° C.) and a bottom layer of 10 μm PET-IA (MP about 150°C.).

*5: PET-IA is polyethylene terephthalate/isophthalate copolymer.

*6: PET is polyethylene terephthalate.

*7: PP is polypropylene.

*8: PE is polyethylene.

EXAMPLE 2

Table 3 shows examples of the present invention. The two types of filmsof the materials shown in the table were simultaneously laminated on thetwo sides of steel sheets. MP1 indicates the melting point of the highmelting point film, d1 indicates its thickness, MP2 indicates themelting point of the low melting point film, and d2 indicates itsthickness. The sheet temperature Ti indicates the metal sheettemperature at the time of lamination, while the speed v indicates thesheet running speed at the time of lamination. Hz1 and Hz2 were measuredby the method explained above based on JIS-K7136. Further, Table 4 showscomparative examples. TABLE 3 Evaluation result Sheet Can Can insidesurface temper- PET inside Corrosion Can outer surface side side atureSpeed surface surface resistance Material of MP1 d1 Material MP2 d2 Ti vHz1 Hz2 ΔHz x-ray Form- whit- after film (° C.) (μm) of film (° C.) (μm)° C. m/s % % % Ia/Ib*1 ability ening working*2 Inv. PET-IA*5 226 20 PP*7 168 30 225 3.5 28.9 32.8 3.9 2.6 Good Very No abnor- Ex. 9 goodmality Inv. PET-IA*5 226 20 PP *7 168 25 233 2.9 29.5 38.9 9.4 1.5 GoodVery No abnor- Ex. 10 good mality Inv. PET-IA*5 226 20 PP *7 168 30 2262.9 27.6 43.5 15.9 4.6 Good Very No abnor- Ex. 11 good mality Inv.PET-PBT*3 214 12 PP *7 168 25 212 2.9 25.3 44.3 19 4.8 Good Good Noabnor- Ex. 12 mality Inv. PET*6 265 20 PP *7 168 40 271 3.5 38.6 42.33.7 1.4 Good Very No abnor- Ex. 13 good mality Inv. PET-IA*5 226 20 PP*7 168 40 233 2.5 35.2 44.8 9.6 1.5 Good Very No abnor- Ex. 14 goodmality Inv. PET-IA*5 226 13 PP *7 168 40 228 2.5 34.3 48.9 14.6 3.2 GoodVery No abnor- Ex. 15 good mality Inv. PET-PBT*3 214 12 PP *7 168 40 2162.5 32.6 50.2 17.6 3.9 Good Good No abnor- Ex. 16 mality Inv. PET-IA*5226 20 PE*8 112 80 257 2.5 48.5 50.8 2.3 0.7 Good Very No abnor- Ex. 17good mality Inv. PET*6 265 13 PP *7 168 70 285 2.5 46.2 55.9 9.7 1.2Good Very No abnor- Ex. 18 good mality Inv. PET-IA*5 226 13 PE*8 112 80242 2.5 46.8 61.6 14.8 2.9 Good Very No abnor- Ex. 19 good mality Inv.PET-IA*5 226 13 PP *7 168 85 226 2.5 47.5 67.2 19.7 4.5 Good Good Noabnor- Ex. 20 mality Inv. 2-layer PET*4 to 20 PP *7 168 20 207 2.5 19.724.9 5.2 8.9 Good Very No abnor- Ex. 21 150 good mality Inv. 2-layerPET*4 to 20 PP *7 168 20 195 2.5 17.6 27.6 10 9.2 Good Very No abnor-Ex. 22 150 good mality Inv. 2-layer PET*4 to 20 PP *7 168 20 183 2.515.8 31.6 15.8 9.6 Good Very No abnor- Ex. 23 150 good mality Inv.2-layer PET*4 to 20 PP *7 168 20 172 2.5 14.6 34.3 19.7 10 Good Good Noabnor- Ex. 24 150 mality Inv. 2-layer PET*4 to 20 PP *7 168 15 188 2.59.9 16.3 6.4 9.4 Good Very No abnor- Ex. 25 150 good mality Inv. 2-layerPET*4 to 20 PP *7 168 15 178 2.5 8.8 20.1 11.3 9.8 Good Good No abnor-Ex. 26 150 mality Inv. 2-layer PET*4 to 20 PP *7 168 15 168 2.5 6.9 25.919 12.5 Good Good No abnor- Ex. 27 150 mality Inv. PET*6 265 13 PP *7168 90 285 2.5 58.7 66.8 8.1 1.6 Good Very No abnor- Ex. 28 good malityInv. PET*6 265 13 PP *7 168 80 287 2.5 52.1 57.3 5.2 1.4 Good Very Noabnor- Ex. 29 good mality Inv. PET-IA*5 226 20 PE*8 112 90 247 2.5 59.273.2 14 2.3 Good Good No abnor- Ex. 30 mality

TABLE 4 Sheet Evaluation result Can inside surface temper- PET CanCorrosion Can outer surface side side ature Speed surface insideresistance Material of MP1 d1 Material MP2 d2 Ti v Hz1 Hz2 ΔHz X-rayForm- surface after film (° C.) (μm) of film (° C.) (μm) ° C. m/s % % %Ia/Ib*1 ability whitening working*2 Comp. PET-PBT*3 214 12 PP *7 168 30206 2.9 25.1 53.8 28.7 4.8 Poor Poor Medium Ex. 6 corrosion Comp.PET-PBT*3 214 12 PP *7 168 35 206 2.9 28.6 60.6 32 4.9 Poor Poor MediumEx. 7 corrosion Comp. PET-PBT*3 214 12 PP *7 168 50 216 2.5 33.5 63.830.3 4.2 Poor Poor Small Ex. 8 corrosion Comp. PET-PBT*3 214 12 PP *7168 40 206 2.5 31.2 55.3 24.1 5.2 Poor Poor Small Ex. 9 corrosion Comp.PET-IA*5 226 13 PP *7 168 90 226 2.5 49.8 72.3 22.5 4.8 Poor Poor SmallEx. 10 corrosion Comp. PET-IA*5 226 13 PE*8 112 80 226 2.5 43.6 68.725.1 4.3 Poor Poor Small Ex. 11 corrosion Comp. 2-layer to 20 PP *7 16825 172 2.5 14.4 39.8 25.4 10.8 Poor Poor Large Ex. 12 PET*4 150corrosion Comp. 2-layer to 20 PP *7 168 20 150 2.5 13.8 49.6 35.8 12.5Poor Poor Large Ex. 13 PET*4 150 corrosion Comp. 2-layer to 20 PP *7 16815 150 2.5 6.2 38.9 32.7 12.3 Poor Poor Large Ex. 14 PET*4 150 corrosionComp. PET-IA*5 226 13 PE*8 112 90 228 2.5 57.3 78.6 21.3 3.1 Poor PoorSmall Ex. 15 corrosion

In these tables, “Good” in the formability column shows that no filmdamage could be recognized when performing the work of the later *2,while “Poor” indicates that cracks accompanied with whitening wereobserved. “Very good” in the can inside surface whitening columnindicates no whitening at all, “Good” indicates slight whitening of asubstantially harmless extent, and “Poor” indicates that clear whiteningwas observed. Note that *1 to *8 in the evaluation columns of Table 3and Table 4 indicate the same content as in Example 1.

*1: Ia/Ib is the ratio of the following two peaks obtained by X-raydiffraction measurement using CuKα-rays at a two-sided laminated metalsheet at the high melting point film (polyester film) covered side. Iais the X-ray diffraction intensity by the diffraction face of the (100)face parallel to the surface of the polyester film (distance betweenfaces about 0.34 nm), while Ib is similarly the X-ray diffractionintensity by the diffraction face of the (110) face (distance betweenfaces about 0.39 nm). In the examples, Ia/Ib is 0.7 to 10 in range andshows that the crystal orientation remains without complete melting upto the surface layer.

*2: The corrosion resistance after working is the result from punching atwo-sided laminated metal sheet into a disk shape of a diameter of 158mm, drawing it by a draw ratio of 1.56 so that the low melting pointfilm becomes the inner surface to obtain a shallow drawn cup, thenredrawing it by a draw ratio of 1.23 to obtain a can of a cup diameterof 82 mm and a cup height of 52 mm (DRD can), filling the inside with 2%citric acid, then storing it at 37 degrees for six months and examiningthe inside surface of the can for the state of corrosion.

*3: PET-PBT is a mixed resin of polyethylene terephthalate andpolybutylene terephthalate.

*4: Two-layer PET is two-layer structure film with a surface layer of 10μm PET (MP 265° C.) and a bottom layer of 10 μm PET-IA (MP about 150°C.).

*5: PET-IA is polyethylene terephthalate/isophthalate copolymer.

*6: PET is polyethylene terephthalate.

*7: PP is polypropylene.

*8: PE is polyethylene.

As clear from Table 3 and Table 4, the two-sided laminated metal sheetsof the present invention where the difference ΔHz of the haze values ofthe low melting point film before and after bending is made 20% or lessare superior in formability and corrosion resistance after working andare completely free of or exhibit almost no whitening when worked into acan or other container. As opposed to this, in the comparative exampleswhere the difference ΔHz of the haze values before and after bending isover 20%, whitening is observed when worked into a container and theformability and corrosion resistance after working are also inferior, itis learned.

INDUSTRIAL APPLICABILITY

In the laminated metal sheet according to the present invention, it ispossible to adjust the thickness of the low melting point side film soas to simultaneously laminate two types of films with different meltingpoints on the two sides of a metal sheet without sticking to thelamination rolls.

Further, even when bending the sheet to produce a food can etc., thereis no whitening of the low melting point resin film and the inside ofthe can no longer appears to contain foreign matter. Further, if usingfor the high melting point film a biaxially stretched polyester-basedresin film and leaving crystal orientation at that surface layer, thehigh melting point film can be given sufficient hardness or strength andthe film thickness can be made thinner and the cost further reduced.

For this reason, there is the advantage that existing laminationapparatuses can be used as they are.

1. A laminated metal sheet obtained by laminating two types of filmincluding a high melting point film and low melting point film on thetwo sides of a metal sheet, said laminated metal sheet characterized byhaving a low melting point film thickness d2 defined by the followingequation 1:d2(μm)≧k(ΔMP−ΔT)/V where, ΔMP=MP1−MP2 MP1: melting point of high meltingpoint film (° C.) MP2: melting point of low melting point film (° C.) k:constant determined by low melting point film's heat conductivity, heatcapacity*, and temperature, k≧2 [μm·m/(° C.·s)] 0<ΔT=MP1−Φ·Ti≦50 (° C.)Ti: metal sheet temperature at lamination roll inlet side (° C.) Φ:constant determined by heat removal conditions at time of lamination(0.75≦Φ<1) V: sheet running speed (m/s)
 2. A laminated metal sheet asset forth in claim 1, characterized in that said high melting point filmis comprised of polyester-based resin and said low melting point film iscomprised of a polyolefin-based resin.
 3. A laminated metal sheet as setforth in claim 2, characterized in that said polyester-based resin isselected from polyethylene terephthalate, a polyethyleneterephthalate/isophthalate copolymer, polyethylene naphthalate, apolyethylene terephthalate/naphthalate copolymer, a mixed resin ofpolybutylene terephthalate and polyethylene terephthalate, a mixed resinof polybutylene terephthalate and a polyethyleneterephthalate/isophthalate copolymer, and further one of these resinsincluding a pigment or dye.
 4. A laminated metal sheet as set forth inclaim 2, characterized in that said olefin-based resin is selected froma polypropylene, polyethylene, a polypropylene/polyethylene randomcopolymer, or these resins including a pigment or dye.
 5. A laminatedmetal sheet as set forth in claim 1, characterized in that a differenceΔHz between a haze value Hz2 of a low melting point film after bending asheet of a thickness of 1 mm by 180 degrees across said high meltingpoint film side and a haze value Hz1 of the low melting point filmbefore bending is made not more than 20%.
 6. A laminated metal sheet asset forth in claim 5, characterized in that the haze value Hz1 of thelow melting point film before bending is made not more than 60%.
 7. Alaminated metal sheet as set forth in claim 1, characterized by usingfor said high melting point film a biaxially stretched polyester-basedresin film and leaving crystal orientation at that surface layer.
 8. Alaminated metal sheet as set forth in claim 1, characterized in that,among the two types of film including said high melting point film andlow melting point film, when the melting points inside the films differaccording to the position in the thickness direction, the lower one ofthe surface melting point of one film at the side contacting the roll isdesignated as MP2 and the film melting point of the other film at themetal sheet side is designated as MP1 for calculation and lamination. 9.A laminated metal sheet as set forth in claim 8, characterized in thatthe sheet running speed V defined in (1) is made 1 to 3.5 m/s forlamination.